Switching mode amplifiers, which are sometimes referred to as “Class-D” amplifiers, are incorporated as signal amplifiers in many applications, particularly in applications that value efficient electrical power utilization. Applications incorporating switching mode amplifiers include loudspeaker amplifiers, ultrasonic transducer drivers, motor speed controllers, LED brightness controllers, and many other amplifying or output voltage controlling applications. Switching mode amplifiers operate by rapidly switching an output element, such as a transistor, between a conducting and non-conducting state to create a pulsed output waveform. The output element is usually switched with a frequency that is much greater than the highest frequency component of the signal being amplified or produced. The duration for which the output element is configured to conduct relative to the duration for which the output element is configured to not conduct controls a short term average for the voltage delivered to a load by the amplifier. Low pass filtering to remove the on-off switching components is used to produce the desired, lower frequency signal represented by the pulse width durations. In one example, the load incorporates reactive components, including energy storage devices such as inductors, capacitors, or any other electrical structures, to perform a voltage averaging function and implement this low frequency filtering to allow the load to operate with only the short term average of output voltage. The on/off characteristic of switching mode amplifiers generally result in high power efficiency, particularly when compared to amplifiers with active components that amplify signals using a linear operating mode.
The present description describes circuits that include one or more reactive components. It is clear that the described reactive components are able to be strictly reactive, substantially reactive, or exhibits impedance with a substantial reactive component. In an example of an inductor, most real inductors have a reactive component along with a resistive component, where the resistive component may be negligible or small. A component is able to be characterized as substantially reactive if it has a reactive component, i.e., an imaginary inductive component, that results in a phase shift of time varying electrical current flowing though the device as a function of time varying voltage across the device. Such a component introduces a phase shift between voltage and electrical current applied to the device without regard to the resistance of the device. Components with impedance values including imaginary numbers are considered to be substantially reactive without regard to the value of the impedance value's real number component.
Although switching mode amplifiers are generally highly efficient in converting supply power into the desired output signal, the efficiency of switching mode amplifiers has been noted to decrease as the output power of the output signal is reduced. When producing an output signal at a small fraction of the total output power capacity of a switching mode amplifier, very short duration output pulses are generated and the amount of power consumed by the amplifier that is delivered to an output load decreases.
Therefore, the efficiency and performance of switching mode amplifiers are limited by inefficient energy conversion from power supply to output signal when the amplifiers are operated over a large dynamic range.